This invention relates to a process and associated apparatus for the catalytic dehydrogenation of a C.sub.2+ paraffinic hydrocarbon charge. It relates more specifically to the separation of a hydrogen-rich gas and a hydrocarbon liquid phase from effluents of a low pressure reaction containing the hydrogen intended to be recycled.
The prior art is illustrated by U.S. Pat. Nos. 4,381,417; 4,381,418; and U.S. Pat. No. 4,663,493.
It is known that a number of industrial processes using low pressure catalytic reactions operate in a hydrogen environment in which the partial pressure of hydrogen is maintained by recycling of a hydrogen-rich gas contained in a reaction effluent and which has been separated from the hydrocarbons.
This is the case, in particular, for the catalytic dehydrogenation process of LPG's containing propane, butane, and isobutanes to produce monoolefins which serve as intermediates for the production of fuels with high octane number. In the case of the dehydrogenation of isobutane, the isobutene produced can react with methanol to produce methyl tert-butyl ether, an additive that can be used in gasolines.
The prior art is also illustrated by U.S. Pat. Nos. 4,381,418 and 4,381,417. In such processes, the reaction is performed in a continuously regenerated catalytic reactor operating at very low pressure (slightly greater than the atmospheric pressure or under vacuum) and at temperatures of 500.degree.-600.degree. C.
The combination of the recycled hydrogen and the produced hydrogen provide a sufficient partial pressure of hydrogen so as to inhibit the formation of coke, thereby maintaining the stability of the catalyst. Thus, a satisfactory conversion at a higher range of temperatures up to, for example, about 600.degree. C., can be achieved. Generally, the low pressure effluent discharged from the dehydrogenation reaction zone is first cooled by heat exchange against the gaseous charge and then with water at a suitable temperature. The vapor pressure of the effluent is then raised in conventional compression equipment to a higher pressure, which makes possible the separation of the hydrogen and the hydrocarbon compounds of the effluent.
The separation of the hydrogen from the hydrocarbons in the effluent is performed, in general, at a pressure higher than that prevailing in the reaction zone. Moreover, to condense the hydrocarbons in the gas mixture constituting the effluent and containing hydrogen, it is necessary to cool the gas mixture to a temperature lower than that which conventional cumbersome air or water heat exchangers can achieve.
Other recommended external cooling systems employ propane or propylene cycles, but they are expensive and consume large amounts of energy.
Furthermore, since cooling below 0.degree. C. is required for the separation of the hydrogen and hydrocarbon compounds, the water present in the compressed effluent must be removed sufficiently so that the residual water does not freeze, thereby avoiding fouling of the cooling equipment. For this purpose, 3 .ANG.(1 .ANG.=1.times.10.sup.-10 m) molecular sieves are used to remove the water of the effluent before the cooling step.